The subject matter disclosed herein relates to dissolved gas analysis using spectroscopy. Specifically, the subject matter relates to accurate measurement of gas concentrations using wavelength modulation spectroscopy.
Electrical equipment such as transformers use fluids such as castor oil, mineral oil and synthetic oils for insulation purposes. The parameters of the fluid are indicative of incipient faults in the electrical equipment. The parameters of the fluid among other things include information of total combustible gas (TCG). Examples of total combustible gas include carbon monoxide, carbon dioxide, hydrocarbons, oxygen and nitrogen. Specifically, carbon monoxide and carbon dioxide increase in concentration with thermal aging and degradation of insulation of the electrical equipment. Furthermore, hydrocarbons such as acetylene and ethylene increase in concentration due to dielectric breakdown caused due to corona and arcing. Further, concentrations of oxygen and nitrogen are indicative of condition of a gas pressurizing system of the equipment. Therefore, technique of dissolved gas analysis (DGA) is employed to determine the concentration of the gas components in the fluid used in the electrical equipment to predict the incipient fault.
Methods of analyzing the gas concentrations from the fluid-samples extracted from the equipment employ spectroscopic techniques. Conventional spectroscopic techniques include off line DGA techniques and absorption spectroscopy based techniques. However, off line DGA techniques are affected by uncertainties issues. Furthermore, techniques employing direct absorption spectroscopic signals are less sensitive with poor signal-to-noise ratio. In general, conventional spectroscopic techniques determine the concentrations of the gas component in a limited range of concentrations and are based on modulating the light beam to improve the sensitivity of detection. Measurements from spectroscopic techniques such as wavelength modulation spectroscopy are affected by the ambient conditions of the gas, especially the ambient pressure conditions of the dissolved gas.
Furthermore, long path lengths may be needed to increase signal to noise ratio in absorption spectroscopy. This is achieved by using multi-pass cells (MPCs), where a laser beam bounces off between two mirrors in a cavity containing the gas mixture. However, typical MPCs may be complex, bulky and expensive due to usage of multiple optical elements and multiple lasers.